Tunnel boring is orders of magnitude more expensive than railway construction, at typically ~a few hundred million dollars per mile vs ~a few million dollars per mile. I know Musk has plans to magically make tunnel boring significantly cheaper but you need two orders of magnitude cost reduction to make the tunnel digging not cost more than an entire above ground railway.

Cost references: https://tunneltalk.com/TunnelTECH-Apr2015-Arup-large-diamete...

https://www.compassinternational.net/railroad-engineering-co...